The 25-year-old has been in superb form for Mauricio Pochettino's charges this season, notching four times in 18 Premier League appearances.
His performances have seen him recognised on the international stage with Roy Hodgson calling him up for England's friendlies with Chile and Germany in November.
"I can't express how happy I am at the club," he said. "I am so settled. Settled at home with a young child and my family live down here.
"I don't think I am too comfortable either, I get challenged week in week out playing in the Premier League. It is where you want to be as a player.
"I am delighted and as happy as I have ever been here playing for Southampton with one of the best managers in the league.
"I learnt my trade in League One and the Championship and feel I have grown with the club. I am delighted and as happy as I have ever been here playing for Southampton with one of the best managers in the league."
